This invention relates to a gate drive circuit for a MOS power transistor, and more particularly to a low cost gate drive circuit having an isolated output stage.
Various circuits, including many power supply and motor control circuits, utilize one or more MOS transistor switches (MOSFETs or IGBTs) to selectively couple a load to a power supply. In general, one terminal of the power supply is designated as the circuit common or ground, and transistors that couple the load to the circuit ground are referred to as low-side switches, whereas transistors that couple the load to the other terminal of the power supply are referred to as high-side switches. Since the emitter or source of a low-side transistor is referenced to circuit ground, its gate drive circuit may also be referenced to ground, and a simple and inexpensive circuit design may be used. However, the emitter or source of a high-side transistor floats with respect to ground, and its gate drive circuit must therefore be isolated from ground. Drive circuit isolation may be accomplished in several different ways, but all are expensive, and the gate drive circuit frequently costs more than the power transistor it controls. Accordingly, what is needed is a simple and inexpensive isolated gate drive circuit.
The present invention is directed to an improved low-cost gate drive circuit powered by a DC source voltage, the drive circuit having an isolated output stage with a parallel-connected by-pass capacitor and a switched input capacitor circuit that maintains a charge on the by-pass capacitor for driving the gate of a controlled MOS transistor while isolating the by-pass capacitor from the source voltage. In a fully isolated embodiment of the improved gate drive circuit, a bank of controlled switches alternately couples the input capacitor to the source voltage and the by-pass capacitor, while in another embodiment, unidirectional isolation is achieved by replacing one or more of the controlled switches with diodes.